bpf: Lift hashtab key_size limit

Currently key_size of hashtab is limited to MAX_BPF_STACK.
As the key of hashtab can also be a value from a per cpu map it can be
larger than MAX_BPF_STACK.

The use-case for this patch originates to implement allow/disallow
lists for files and file paths. The maximum length of file paths is
defined by PATH_MAX with 4096 chars including nul.
This limit exceeds MAX_BPF_STACK.

Changelog:

v5:
 - Fix cast overflow

v4:
 - Utilize BPF skeleton in tests
 - Rebase

v3:
 - Rebase

v2:
 - Add a test for bpf side

Signed-off-by: Florian Lehner <dev@der-flo.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201029201442.596690-1-dev@der-flo.net

1 files changed, 2 insertions, 1 deletions

diff --git a/tools/testing/selftests/bpf/test_maps.c b/tools/testing/selftests/bpf/test_maps.c
index 0d92ebcb335d..0ad3e6305ff0 100644
--- a/tools/testing/selftests/bpf/test_maps.c
+++ b/tools/testing/selftests/bpf/test_maps.c
@@ -1223,9 +1223,10 @@ out_map_in_map:
 
 static void test_map_large(void)
 {
+
 	struct bigkey {
 		int a;
-		char b[116];
+		char b[4096];
 		long long c;
 	} key;
 	int fd, i, value;